Electrodeposition of zinc



Patented June 10, 1941 mcrnopsrosmou or zmc John L. Bray and Facnndo mu Mortal, west Lafayette, Int, assignorsto Purdue Research 1 Foundation, La Fayette, Ind., a corporation of No Drawing.

60laims.

This invention relates to the electrodeposition of zinc and has for its principal object the provision of an improved electroplating bath or composition and an improved electroplating method.

According to our improved method, we are able to produce bright deposits zinc without any tendency to coarseness of crystalline structure or to the production of trees. The depositsthus produced, even when built up to substantial thickness, are tenacious and ductile and do not check or crack when the base metal is bent.

Our improved bath is 0! the acid zinc type but is characterized by addition agents which when present even to a slight extent have the eiiects or decreasing the crystal size 01 the deposited zinc, insuring the production or a continuous coating, reducing any tendency to, roughness or Application January 10, 1938, Serial No. 184,247

(on. ans-5s) treeing, and increasing the throwing power 01' thesolution or bath. Our-zinc deposits have a remarkable resistance to corrosion, which we atzinc deposited.

The addition agents we employ comprise the class of unsubstituted alkylamines. We may use mono-, diand tri-alkylamines. It must be understood that we do not intend to include in the term "unsubstituted alkylamine" any amine which includes a cyclic or aromatic radical since the presence of sucha radical has a very objectionable roughening eii'ect on the deposit. For example, aniline, 11' used as an addition agent, invariably results in the production of rough deposits. Furthermore, the present invention does not contemplate the employment of alkylamines in which one or more oi the hydrogens of the alkyl groups is substituted by an inorganic element or radical.

-Among the unsubstituted alkylamines which may be employed are monomethylamine, dimethylamine, trimethylamine, the corresponding ethyl, propyl, butyl, amyl, and otheralkyl homologues. and also mixed unsubstituted alkylamines such as monomethylmonoethylamine, monomethyldiethylamine, methylpropylbutylamine and so forth. Many 0! these are prohibitive. because of cost or diiiiculty of obtaining them. In actual practice, we prefer to employ trimethylamine on account of its lowcost and its high emciency.

go tribute to the great purity and smoothness oi the sulphate and a small amount of free acid. The

alkylamine may. be present in very slight amount, its action being apparently oi. a catalytic nature.

We give the following examples of baths embodying our invention:

Example I:

Zinc grams per liter Excess sulphuric acid '.i (Ml-2.0% Trimethylamine (25% by weight (CHshN) aqueous sclutioni0.0002-0.005 cc. per liter Example II: f Zinc 200 grams per liter Excess sulphuric acid (Ll-2.0% Trimethylamine by weight (CHshN') aqueous solution--- 0.000025-0.005 cc. per liter The composition of our electrolyte may be varied within wide limits. Thus the specific gravity may suitably be between 1.23 and 1.46 corresponding to between 1.7 and 3.5 pounds of zinc ions per gallon.

Care must be taken to prevent mechanical loss of trimethylamine owing to so-called "drag," that is owing to-bath liquid being carried'away with the plated metal. Thus in plating wire we prefer to add about one ounce 0! pure trimethylamine for each ton of cathode zinc deposited.

In electroplating with zinc according to our invention, we may employ various current densities, for example, between 350 and 2,000 amperes per square foot and obtain extremely, pure zinc deposits 01' highly desirable mechanical properties.

We claim:

i. A bath for the electrodeposition of zinc comprising an acid solution of zinc sulphate and an addition agent-adapted to act as a catalyst and consisting of an unsubstituted alkylamine oi the character described in an amount substantially equivalent to that supplied within the range of 0.000025-0.005 cc. per liter (25% by weight (cnmm aqueous solution;

2. A bath for the electrodeposition of zinc com- The bath employed preferably contains flnc prising. an acid solution of zinc sulphate and an 2 d addition agent adapted to act as a. catalyst and consisting of an unsubstituted trimethyle in an amount substantially equivalent to that supplied within the range of 0.000025-0.005'cc. per liter (25% by weight (CHalaNl aqueous solution.

3. A bath for the electrodeposition of zinc comprising the following constituents within substantially the ranges indicated:

Zinc 100-200 grams per liter Excess sulphuric acid. 0.01-2'.0% Trimethylamine (25% by weight (CHaMN) aqueous solution--- 0.000025-0.005 cc. perliter 4. The method of electroplating zinc which comprises the passing of an electric current ranging in density substantially between 350-2000 amperes 'per square foot through a bathcomprising an acid solution of zinc sulphate and an addition agent adapted to act as a catalyst and consisting of an unsubstituted alkylamlne of the character described in an amount substantially equivalent to that supplied' within the range 0.000025-0 .00 5 cc. per liter (25% by weight (cumin e ous solution.

.5. The method-pf electroplating zinc which comprises the passing of aneiectric current ranging in density-substantially between 350-2000 amperes per square foot through a bath comprising an acid solution of 'zinc sulphate and an addition agent adapted to act as a catalyst and consisting of an unsubstituted trimethylamine inv an amount substantially equivalent to that supplied within the range of 0.000025-0005 cc. per liter by weight (CH3):N) aqueous solution.

8. The method of electroplating zinc which comprises the passing of an electric current ranging in density substantially between 350-2000 amperes per square foot through a bath comprising-constituents within substantially the ranges indicated: zinc -200 grams per liter, excess sulphuric acid OBI-2.0%, trimethylamine (25% by weight (CH3)3N) aqueous solution 0.000025- 0.005 cc. per liter.

. JOHN L. BRAY.

F. ROLF MORRAL. 

